In one type of laser printer, a photosensitive material is supported on a rotatable drum, and a print head carrying a light source (e.g., a laser) is advanced relative to the photosensitive material by means of a lead screw. The light source is modulated in accordance with an information signal to form an image on the photosensitive material. In order to increase the output of such apparatus, multiple light sources (e.g., lasers) are mounted in the print head to form multiple channels so that a plurality of print lines can be formed in a single pass. In multiline scanning systems, any difference of densities among the lines can create very severe artifacts in the image. These artifacts can appear as repetitive patterns known as "banding."
The problem of banding can be particularly troublesome in half-tone printing where, for example, 12 mini-pixels are used to write a half-tone dot. Visible lines in the image, caused by unevenness in the densities of the lines, can come at a different section of each successive half-tone dot, and thus cycle across the image. The visible lines can be due, for example, to a variation in the intensity of the light sources (e.g., the lasers). The frequency of the visible lines in the image beats with the half-tone dot frequency. The resulting macro density variation can have a spatial frequency in the image which, unfortunately, matches the frequency at which the human eye is most sensitive, that is, at about 0.5 cycle/mm. At this frequency range, the typical human eye can see a variation of density of around 0.2 percent. This small level of unevenness in density is difficult to control in a printer using a multiline print head.
Laser power measurements are used in a multichannel printer like the one disclosed in the above mentioned U. S. Patent Application to track individual laser performance over time. Individual laser currents can then be adjusted to maintain constant power levels, which produce equal amounts of print density within one swath. A swath is created by the number of writing lines printed during one revolution of the drum. Without such adjustments, the differences in output among the several channels of the printer would eventually produce unacceptable results. The imbalance within the swath would, in other words, produce visible artifacts and the resulting print would become unusable.
In the past, laser performance measurements have been made by positioning the print head in front of an integrating sphere. A silicon sensor mounted in the integrating sphere was amplified and the output signal sent to an analog to digital converter. The power for each channel was then recorded at as many as 33 different settings. During each of these settings the laser was turned on for 3 seconds. Over 100 readings were measured, and the difference between the average value and the value with no laser on was saved. A spline curve was fit to the resulting set of 33 readings for each channel.
This earlier method tended to produce an undesirable amount of variability from one calibration run to the next. A number of problems have been observed. When the laser is used to sublimate dye it is not normally turned on at a 100 percent duty cycle. Differences between pulsed and continuous operation may be large enough to account for some of the discrepancies between measured power and resultant density. In addition, laser output tended to vary as a function of temperature. Using a linearly incrementing list of test levels increases the local temperature of the laser and the calibration sensor during the time of the test. As a result, the results are biased and a hysteresis effect is introduced. A spline curve fit also tends to follow the results too closely. The individual data points can appear to contain step discrepancies which may not always occur in the same place but, rather, are a function of the history of how the laser was operated. This latter effect appears to be due to the different rates of heat generation and dissipation within the laser package. A spline curve fit tends also to be sensitive to intermittent noise during the calibration process. For all of these reasons, a better way of measuring laser performance would clearly be beneficial.